Generally, hydraulic power steering apparatuses have been used as power steering apparatuses for vehicles. Since the 1990's, electric power steering apparatuses using a motor have been popularized.
In a conventional hydraulic steering apparatus, a hydraulic pump, which is a power source providing assistant steering power, is operated by an engine, and always consumes energy regardless of the rotation of a steering wheel. On the other hand, in an electric power steering apparatus, if torque is generated by rotation of a steering wheel, a motor provides assistant steering power proportionate to the generated steering torque. Therefore, when the electric power steering apparatus is used, it is possible to improve energy efficiency in comparison with the hydraulic power steering apparatus.
FIG. 1 is a block diagram of an electric power steering apparatus according to the conventional art.
As shown in FIG. 1, in general, an electric power steering apparatus includes a steering system 100 extending from a steering wheel 101 to each wheel 108 on both sides of the vehicle, and an assistant power mechanism 120 providing assistant steering power to the steering system.
The steering system 100 has an upper part connected to the steering wheel 101 to rotate along the steering wheel 101, and a lower part connected to a pinion shaft 104 by a pair of universal joints 103. Further, the pinion shaft 104 is connected to a rack bar 109 through a rack-pinion mechanism 105, while both ends of the rack bar 109 are connected to both wheels 108 of the vehicle by tie rods 106 and knuckle arms 107, respectively.
The rack-pinion mechanism 105 includes a pinion gear 111 formed on a lower part of the pinion shaft 104, and a rack gear 112 formed on a peripheral surface of one end of the rack bar 109 and engaged with the pinion gear 111. The rotational movement of the pinion shaft 104 is changed to a linear movement of the rack bar 109 through the rack-pinion mechanism 105.
Therefore, when a driver operates the steering wheel 101, the pinion shaft 104 rotates, the rack bar 109 straightly moves along the axial direction thereof depending on the rotation of the pinion shaft 104, and the linear movement of the rack bar 109 causes the tie rods 106 and knuckle arms 107 to steer the wheels 108.
The assistant power mechanism 120 includes a torque sensor 121 for sensing steering torque applied to the steering wheel 101 by the driver and outputting an electric signal in proportion to the sensed steering torque and an electronic control unit 123 for generating a control signal depending on the electric signal transmitted from the torque sensor 121. The assistant power mechanism 120 also includes a motor 130 for generating assistant steering power according to the control signal transmitted from the electronic control unit 123, and a belt-type power transmission apparatus 140 for transferring the assistant power generated by the motor 130 to the rack bar 109 through the belt.
Accordingly, in the electric power steering apparatus, the steering torque generated by the rotation of the steering wheel 101 is transferred through the rack-pinion mechanism 105 to the rack bar 109. The assistant steering power, which the motor 130 generates depending on the generated steering torque, is transferred by the belt-type power transmission apparatus to the rack bar 109. In other words, the torque generated in the steering system 100 and the assistant steering power generated by the motor 130 are combined with each other so as to cause the rack bar to be moved along an axis thereof.
Meanwhile, the electric power steering apparatus may have a structure that the assistant steering power generated in the motor 130 is transferred to the steering wheel 102 or the pinion shaft 104.
FIG. 2 is a sectional view illustrating the conventional electric power steering apparatus.
As shown in FIG. 2, the conventional electric power steering apparatus for the vehicle includes the rack bar 109 extending in a transverse direction of the vehicle and having the rack gear in an outer peripheral surface of an end thereof, the pinion shaft 104 having the pinion gear engaged with the rack gear. The conventional electric power steering apparatus includes a ball screw part 210 having a ball nut 205 engaged with a ball screw 203 through balls 201, the belt-type power transmission apparatus 140 connecting a ball nut 205 to the shaft 221 of the motor, and the motor 130.
The pinion shaft 104 is connected to the steering wheel through the steering shaft, and the rack bar 109 having the screw formed by a predetermined length at one end thereof is contained in a rack housing. Furthermore, the rack housing includes the first housing 225 having the rack gear and the second housing 227 having the motor.
The belt-type electric transmission apparatus 140 includes a belt connecting the shaft 221 of the motor 221 and the ball nut 205, and transfers steering assistance power generated in the motor 130 in proportion to the steering toque applied to the steering wheel to the rack bar 109 through the ball nut 205.
However, the belt-type electric transmission apparatus 140 enables vibration and impact, etc. of the rack bar 109 not to be directly transferred to the motor 130. However, as the intension of the belt is changed due to the wear of the belt or a pulley, vibration is generated in the belt and can be transferred to the body of the vehicle, thereby causing noise.